Electrode assembly for cathode-ray tubes



April 3, 1951 M. SILVERMAN 2,547,415

ELECTRODE ASSEMBLY FOR CATHODE-RAY TUBES Filed Oct. 16. 1948 IN; EN TOR. MAR T/N S/L VERMAN a440, v ATTORNEY STATES ELECTRODE ASSEMBLY FOR CATHODE-RAY TUBES Martin Silverman, Philadelphia, Pa., assignor to TENT OFFICE Electronic Tube Corporation, Philadelphia, Pa., a corporation of Pennsylvania Application Gctober 16, 1948, Serial No. 54,856

This invention relates to an electrode assembly for cathode ray tubes, the assembly being particularly adapted for the securing of desirable characteristics of operation of tubes involving magnetic beam deflection.

In cathode ray tubes involving magnetic deflection of the beam it is desirable to provide an electron beam of 350 microamperes or greater and having a control grid cut-off at minus '7 volts or less. It is also desirable that this tube have a line width of 0.5 millimeters or less when measured at an electron beam current of 200 microamperes under usual conditions. Great difficulty has been involved in securing both of these characteristics. When a high beam current is involved a small line width may be secured but only at the expense of a high cut-oii potential; on the other hand, tubes designed for cut-ofi potentials of the order indicated above involve relatively large line widths.

It is the general object of the present invention to provide an electrode assembly which will produce simultaneously the characteristics indicated above as desirable. In brief, this end is attained by utilizing a control grid in the form of wire mesh, the wire mesh being of approximately semispherical shape and located closely adjacent to the emitting cathode.

The attainment of the foregoing object and of the other objec..s relating to details of construction will become apparent from the following description read in conjunction with the accompanying drawing, the figure in which illustrates in section, on a greatly enlarged scale, a suitable electrode assembly.

The type of cathode ray tube to which the invention is applicable is, in most respects, conventional and the drawing and description will be directed primarily to the assembly of the cathode, the control grid and the second or accelerating grid with the understanding that conventional focusing and accelerating electrodes are associated therewith and deflection is secured through the disposition of deflecting coils in conventional fashion. While the invention is particularly applicable to tubes involving magnetic deflection of the high beam current type it will be evident that the invention is also applicable to tubes involving electrostatic deflection though the problems involved in the former are more easily solved.

Referring to the drawing, there is illustrated at 2 a conventional cathode tube terminating in the radial coated electron-emitting surface 4. Heating is efiected through the use of the con- 3 Claims. (01. 250-27.5)

ventional heater coil indicated at 5. The control grid is in the form of a disea provided with a circular aperture I0 supported by a conventional tube structure. The second grid illustrated at I2 is in the form of a disc at the end of a supporting tube, the disc being provided with a central aperture provided by extrusion as indicated at 14. This aperture at 14 is illustrated as having the same diameter as the aperture IQ of the control grid but it will be understood that this correspondence of dimensions is not essential but may be varied depending upon the dimensions of various associated electrode elements.

The invention primarily involves the association with the control grid of a fine mesh wire screen It which may be spot welded as at l8 to the control grid disc though it will be evident that it may be otherwise secured. This wire mesh is disposed convexly toward the cathode-emitting surface and has the form of an approximately spherical surface closely approaching the emitting surface and closely approaching that of the equipotential contour. Its shape forms, in effect, the effective cathode surface which, from the standpoint of the optics involved, is thus substantially spherical.

While the invention is not limited to particular dimensions it will be illustrative to cite a particular set of dimensions. With control grid and second grid openings of 0.040 inch diameter and with a spacing of the control grid disc 8 from the cathode emitting surface of 0.018 inch and a spacing at their closest points between the grids of 0.007 inch, highly satisfactory results have been obtained by providing a spherical wire mesh having a. radius of curvature of 0.070 inch spaced from the cathode at its closest approach thereto by approximately 0.001 inch. The screen It was, in this case, mesh formed of 0.0016 inch diameter wire. In the case of assemblies having the foregoing and similar dimensions there was obtained an electron beam of upwards of 350 microamperes and a control grid cut-ofi at minus 7 volts or less and also a line Width of the order of 0.5 millimeters at an electron beam current of 200 microamperes. As will be understood by those skilled in the art changes of mesh size involve corresponding changes in other dimensions. Finer meshes may be used but inasmuch as they involve, for a corresponding ratio of opening area to total area, the use of finer wires, and since the wire mesh should very closely approach the cathode-emitting surface difficulties tend to arise .due to sparking between the mesh and the oathode which may result in fusion of the fine Wires.

Such fusion has a detrimental effect on the characteristics and particularly on the size of the crossover and the corresponding line width. The transconductance, however, is somewhat improved by the use of finer mesh.

It will be evident that details of construction are subject to considerable arbitrary choice without departing from the scope of the invention as defined in the following claims.

What I claim and desire to protect by Letters Patent is:

1. An electrode assembly for a cathode ray tube including a cathode having a substantially plane electron-emitting surface and a control grid, said control grid having an aperture therein and a wire mesh screen covering said aperture, each of the openings in said Wire mesh being of substantially smaller area than the aperture, said wire mesh being of approximately semi-spherical shape and convex towards and spaced a distance less than the Wire mesh openings from said emitting surface of the cathode to effect cut-ofi of the electron stream from said cathode at a relatively low negative potential.

2. An electrode assembly for a cathode ray tube including a cathode having an electron-emitting surface and a control grid, said control grid having an aperture therein and a wire mesh screen covering said aperture, each of the openings in said wire mesh being of substantially smaller area than the aperture, and the spacing between said mesh and the center portion of the electron-emitting surface of said cathode being smaller than the wire mesh openings to efiect cut-off of the electron stream from said cathode at a relatively low negative potential and the spacing between said mesh and the portion of said cathode surrounding said center portion being greater than the spacing at said center portion to provide minimum line width of the electron stream.

3. An electrode assembly for a cathode ray tube including a cathode having an electron-emitting surface and a control grid comprising a wire mesh screen of approximately semi-spherical shape and convex towards and spaced a distance less than the wire mesh openings from the center portion of said emitting surface of the cathode to effect cut-off of the electron stream from said cathode at a relatively low negative potential and less closely spaced from the portion of said cathode surrounding the center portion to provide minimum line width of the electron stream.

MARTIN SILVERMAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,079,163 Gardner et a1. May 4, 1937 2,223,040 Mahl Nov. 26, 1940 2,406,850 Pierce Sept. 3, 1946 2,444,749 Pearce et a1 July 6, 1948 

